Omega-3 fatty acids are a family of unsaturated fatty acids with a carbon-carbon double bond at the third bond from the methyl end of the fatty acid. The human body cannot synthesize omega-3 fatty acids de novo. Instead, they are obtained in the human diet from certain fish, such as cod, mackerel, herring, salmon, and sardines. Nutritionally important omega-3 fatty acids include docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). EPA acts as a precursor for prostaglandin-3 (which inhibits platelet aggregation), thromboxane-3, and leukotriene-5. DHA is metabolized to form the docosanoids, which comprise several families of hormones. DHA is also a major fatty acid in sperm and brain phospholipids. High levels of both EPA and DHA have been linked to reduced triglycerides, heart rate, blood pressure, and atherosclerosis. Decreased levels of EPA have been linked to depression and schizophrenia. Decreased levels of DHA have been linked to Alzheimer's disease.
EPA, also sometimes known as timnodonic acid or by the shorthand name 20:5(n−3), is a carboxylic acid with a 20-carbon chain with 5-cis double bonds, the first of which is located at the third carbon from the omega end of the carbon chain. EPA has a molecular mass of approximately 302.451 g/mol.
DHA, also sometimes known as cervonic acid or by the shorthand name 22:6(n−3), is a carboxylic acid with a 22-carbon chain with 6-cis double bonds, the first of which is located at the third carbon from the omega end of the carbon chain. DHA has a molecular mass of approximately 328.488 g/mol.
Quantitation of EPA and DHA by liquid chromatography-mass spectrometry with ESI has been reported. For example Salm, et al., Biomed Chromatogr., 2010, Epub ahead of print, report quantitation of EPA and DHA in plasma by HPLC-ESI (negative ion)-MS/MS; Lacaze, et al., J. Chromatog. A, 2007, 1145:51-57 report methods for quantitating free fatty acids in shellfish tissue extracts with an LC-ESI (negative ion)-MS method; and Zehethofer, et al., Rapid Communications in Mass Spectrom., 2008, 22:2125-33 report quantitating free fatty acids in plasma with an UPLC-ESI (positive ion)-MS/MS method. Other mass spectrometric methods for quantitation of EPA and/or DHA derivatives, metabolites, or oxidation products have also been reported. For example, {hacek over (R)}ezanka, reports quantitation of EPA and DHA by preparation of methyl esters of EPA and DHA followed by detection of the methyl esters by HPLC-APCI (negative ion)-MS. (See {hacek over (R)}ezanka, Tomà{hacek over (s)}, J. High Resol. Chromatogr. 2000, 23:338-42 (EPA and DHA in linseed oil and prepared standards); and {hacek over (R)}ezanka, Tomà{hacek over (s)}, Biochemical Systematics and Ecology 2000, 28:847-56 (EpA and DHA in three freshwater crustacean species.) Additionally, Fer, et al., J. Lipid Research 2008, 49:2379-89, reports detection of ω- and (ω−1)-hydroxylated derivatives of EPA and DHA by an HPLC-APCI (negative ion)-MS method; Yin, et al., J. Biol. Chem. 2007, 282:29890-901, report detection of autoxidation products of EPA and DHA by an HPLC-APCI (negative ion)-MS method; Lawson, et al., J. Lipid Research 2006, 47:2515-24, and Gao, et al., J. Biol. Chem. 2007, 282:2529-37, report detection of oxidized derivatives of EPA and DHA with LC-ESI (negative ion)-MS/MS.